I've tried the first one. He only drops the first ball when he's making the grab for the second. Unfortunately he gets a bit over excited and this usually results in some painful fingers so I had to stop doing it. I'm fast, but he's faster.

Reinforcing it at home is a good idea actually, it's so obvious I never thought about it. When we're not playing and he drops the ball on the laptop, in the bath etc. we're not only not rewarding him for doing so, but sometimes even telling him 'no'. Not that it makes a difference indoors but it might be something he's taking outside.

Sometimes he tries playing in the car, and then it gets confiscated to the glove box before it rolls under the brake pedal. Again this could be reinforcing the opposite message.